An inherent problem, known in the art as "torque ripple," exists in the use of electric motors of the type in which motor commutation is controlled in response to sensed motor rotor position. Motors of this type are commonly used as stepping motors to control the position or orientation of a driven member. When such a motor is continuously energized, the amount of torque produced by the motor varies as a function of the motor's rotor position. This variation in the torque is the motor's torque ripple. The torque ripple is the amount of the fluctuation between a maximum torque and a minimum torque as the motor is driven at a constant rate and constant load. Since the motor is continuously energized, the torque ripple also varies over time.
One particular type of electric motor that exhibits torque ripple during its use is a variable reluctance motor. Variable reluctance motors and their principle of operation are known in the art. These motors are well suited to many applications due to their high torque-to-inertia operating ratio. A limiting factor in the use of a variable reluctance motor for continuous drive systems, however, has been the torque ripple they exhibit during use.
Known systems have utilized various techniques for controlling torque and torque ripple during operation of a variable reluctance motor. For example, U.S. Pat. No. 4,868,477 to Anderson et al., is directed to an arrangement for controlling torque and torque ripple in a variable reluctance motor. In accordance with the '477 patent, a constant current is applied to each motor phase. In response to this applied constant current, a torque waveform as a function of rotor position is generated for each motor phase. A plurality of torque waveforms are generated by applying a plurality of constant current values. The plurality of torque waveforms are then used to determine a table of values. Each value in the table corresponds to the current to be supplied to a motor phase for a given rotor position to achieve the desired motor torque with reduced torque ripple.
U.S. Pat. No. 4,961,038 is directed to an arrangement for estimating torque generated by a switched reluctance motor using a look-up table to generate a torque estimate based upon phase current and rotor position. The estimated torque is used as a torque feedback signal to a controller to modulate motor phase current commands to reduce torque ripple.
A basic problem with known motor control systems is that these systems assume a linear superposition of the torque generated by the individual phase coils of the motor. In effect, these systems assume that the sum of the torque generated by exciting adjacent coils individually is the same as the motor output torque when the two phases are simultaneously excited. Control arrangements that make this assumption often exhibit torque ripple during motor operation.
Another control arrangement for reducing torque ripple in a switched reluctance motor has been proposed in a paper entitled "Control of Switched Reluctance Motor Torque for Force Control Applications" by Andrew A. Goldenberg, Izhak Laniado, Pawel Kuzan, and Chin Zhou, IEEE Transactions On Industrial Electronics, Vol. 41, No. 4, August 1994, pages 461-466. In accordance with the article, the switched reluctance motor torque function is evaluated off-line by (1) commanding a constant current using a proposed commutation algorithm, (2) moving the motor one electrical period, and (3) measuring the torque during this movement. The procedure is repeated with distinct current values from zero to a maximum value so as to get a set of torque curves versus position and current. The set of curves is used to generate a look-up table. Torque versus position and current is smoothed using an inverse torque function. The torque function is estimated by taking the average of the measured torque in two directions and then subtracting friction from the measured torque. Friction is compensated using feed-forward controllers to provide a current correction term derived from the look-up table.